Added support for hg diffs generated in git mode

Summary: Fixes T2112. These are fairly common now, and are used as the storage format for `hg export` and mq in most installs.

Test Plan: Ran unit tests, used `arc patch --patch`, uploaded some diffs manually.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2112

Differential Revision: https://secure.phabricator.com/D4592